Plasma energy is sometimes used to facilitate or activate chemical reactions for thin film deposition. When applied to a gaseous precursor, plasma energy can generate neutral radicals and/or ions, transforming the more stable precursor into an activated species. The energetic activation barrier for a subsequent film forming reaction of the activated species would be expected to be lower than for the precursor. In turn, the deposition reaction should proceed with little need for thermal energy. Eliminating thermal reactions can be advantageous when the reaction temperature can activate undesired alternative reaction paths or alter the mechanical or physical properties of the substrate or other materials formed thereon.
Mass transfer limitations that can cause surface thickness non-uniformity are typically absent in kinetically-limited chemical vapor deposition (CVD) processes (e.g., some atomic layer deposition (ALD) processes). In an ALD process, a substrate is sequentially exposed to different precursor compounds, or precursors. These precursors chemically adsorb, or chemisorb, via chemical reaction with the substrate surface, to form metastable chemisorbed species. Subsequent exposure of these species to a different precursor leads to a surface reaction causing that precursor to become chemisorbed to the surface and incorporated therein, building up the film. In between exposures, non-chemisorbed amounts of the precursors are removed from the surface of the substrate (e.g., by evacuation and/or displacement via purging with a non-reactive gas) so that no more than chemisorbed amounts of one particular precursor is present on the surface during exposure to another precursor. This layer-by-layer exposure creates highly uniform films.
For example, some metal oxide ALD processes use organometallic compounds that may suffer from unwanted decomposition reactions when exposed to elevated temperatures. The metal atoms in these compounds are often stabilized with ligands bound to the metal atom which are broken upon adsorption of the precursor molecule to the surface.
In thermal ALD, surface energy activates all of the bond breaking and forming reactions during chemisorption for all of the precursors used to form the film. Because it is often problematic to vary the substrate temperature between subsequent exposure steps, it can be challenging to find a combination of precursors that will thermally decompose to form chemisorbed species stable enough to remain on the surface and active enough to remain reactive. That is, if the first precursor is too surface-stable, it may block chemisorption and/or reaction of the second precursor; if it is not stable enough, it may decompose.
In plasma enhanced ALD, the plasma supplies the energy to form a reactive intermediate. This intermediate can then react with a chemisorbed precursor to form a layer of film. While offering some advantages relative to thermal ALD, plasma ALD can be problematic, as the identity and concentration of reactive species formed within the plasma can be difficult to control. This can affect the chemical composition and/or the performance properties of the deposited layers.